Abstract: For realizing the accurate feedback control of engine electronic control system on air-fuel ratio and facilitating the diagnosis of engine an air-fuel ratio acquisition-display system and a PC monitor system were developed based on the working principle of universal exhaust gas oxygen (UEGO) sensor. The corresponding relationship between the output voltage and the air-fuel ratio in the system was calibrated. The test systems were also validated on the engine testing bench. The results showed that the systems had high sensitivity and fast response time (100 ms). Meanwhile, the designed systems could show the testing data directly. Those two systems were proposed to be used in the development of engine closed loop air-fuel ratio control and on-board diagnostics (OBD) system.